The present invention relates to an evacuating-type waste water collecting system for collecting the waste water discharged from a number of houses, and more particularly to a vacuum pump control apparatus for such a system.
An evacuating-type waste water collecting system has been known as one of the systems for collecting waste water discharged from a number of houses. FIG. 3 shows the overall structure of such an evacuating-type waste water collecting system. As shown in the figure, the waste water having been discharged from each of the houses 30 (located on the ground surface) flows into an underground cesspool 32 through sewer pipes 31 of the type utilizing a natural downflow. When a predetermined amount of waste water has collected at the lower portion of the cesspool 32, a vacuum-operated valve 33 attached at the upper portion within the cesspool 32 is opened so that the waste water in the cesspool 32 is sucked through a suction pipe 34 together with air which amounts to several times the volume of the waste water. This waste water is then sucked via the vacuum-operated valve 33 into vacuum sewer pipes 35 which are distributed beneath the ground surface, and therefore connected to a water collecting tank 1 at the vacuum pump site 40. Waste water collected in the water collecting tank 1 is then sent to a sewage treatment plant or the like by means of a booster pump 3.
To generate a negative pressure at the interior of the water collecting tank 1 and at the interior of the vacuum sewer pipes 35, a vacuum pump 2 is attached to the water collecting tank 1. Operation of the vacuum pump 2 and the booster pump 3 has conventionally been controlled in response to the gas pressure and the liquid level of the waste water in the water collecting tank 1 respectively. In other words, the vacuum pump 2 is controlled to start when the gas pressure in the water collecting tank has risen above a set value (i.e., towards atmospheric pressure) and to stop when the pressure is less than another set value. Booster pump 3, on the other hand, is controlled in such a manner that it is started when the liquid level of waste water within the water collecting tank 1 has risen above a set value, while it is stopped when the level falls below another set value.
In this type of system, generally, a two-phase flow consisting of gas and liquid occurs within the vacuum sewer pipe 35, and waste water, drawn by the force with which said gas is sucked toward the water collecting tank 1, is also carried to the water collecting tank 1. Thus, it is not possible for a specific portion of the vacuum sewer pipe 35 to be filled only with waste water.
For some unspecified reason, however, a portion of the vacuum sewer pipe 35 with an upgrade toward the vacuum pump site 40 (like portion "A" shown in FIG. 3) may be filled with waste water, causing a so-called air lock. In such a case, the negative pressure generated at the water collecting tank 1 is significantly reduced at the distal ends of the pipe passage of the vacuum sewer pipes 35 (i.e., the air pressure is raised).
When the negative pressure within the pipe is reduced in this way, the amount of air sucked from the vacuum-operated valve 33 is reduced and the gas-liquid ratio in the vacuum sewer pipe (i.e., the amount of air to the amount of the waste water) becomes smaller. In addition, since the volume of the air in the pipe becomes smaller, air locks are caused more easily, thereby resulting in a "vicious cycle" such that the negative pressure is reduced even further at the distal ends of the pipe passage.